Cargo.csv was something used to convert the prices into the format the game uses, but as was said, it is ignored by the engine.
Base prices are given in
master_parts_list.csv. We'll use grain as the example for this. There, its price is given as 13. This is a baseline price. You can adjust that to increase or decrease the price of grain everywhere.
To change how much it goes for on a particular base type, you need to go into
units.csv This is the entry specifying what is bought and sold at all agricultural worlds, and for how much.
{Grain;1.0;0.54;61.75;43}{Generic_Foods;1.0;0.46;63.25;20.5}{Luxury_Foods;1.0;0.32;51.14;16.5}{Furs;1.0;0.07;29.77;9.5}{Liquor;1.0;0.23;0;62}{Pets;1.0;0.17;43.08;16}{Wood;1.0;0.2;59.53;18}{Gems;1.02;0.04;0;0}{Iron;1.26;0.04;0;0}{Tungsten;1.34;0.04;0;0}{Plutonium;0;0;0;0}{Uranium;0;0;0;0}{Artwork;1.03;0.08;0;0}{Games;1.11;0;0;0}{Movies;1.22;0.09;0;0}{Advanced_Fuels;1.03;0.01;0;0}{Communications;1.16;0;0;0}{Computers;1.1;0.03;0;0}{Construction;1.28;0.09;0;0}{Factory_Equipment;1.28;0.09;0;0}{Food_Dispensers;1.17;0;0;0}{Holographics;1.02;0.02;0;0}{Home_Appliances;1.36;0.09;0;0}{Home_Entertainment;1.12;0.1;0;0}{Medical_Equipment;1.03;0.01;0;0}{Mining_Equipment;0;0;0;0}{Plastics;1.54;0.08;0;0}{Pre_Fabs;1.35;0.08;0;0}{Robot_Servants;1.01;0.03;0;0}{Robot_Workers;1.08;0.03;0;0}{Software;1.12;0.05;0;0}{Space_Salvage;1.03;0.05;0;0}{Textiles;1.1;0;0;0}{Weaponry;1.05;0;0;0}{Books;1.1;0;0;0}{PlayThing_(tm);1.22;0.09;0;0}{Contraband/Brilliance;1.24;0.11;0;0}{Contraband/Pilot;1.14;0.05;0;0}{Contraband/Slaves;1.14;0.05;0;0}{Contraband/Tobacco;1.21;0.09;0;0}{Contraband/Ultimate;1.1;0.05;0;0}{starships/Confed/Light;1;;12;5}{starships/Militia/Light;1;;12;5}{starships/Merchant/Light;1;;12;5}{starships/Merchant/Medium;1;;12;5}{starships/Hunter/Light;1;;12;5}{upgrades/Weapons/Mounted_Guns_Light;1;.1;12;4}{upgrades/Weapons/Mounted_Guns_Medium;1;.1;6;3}{upgrades/Weapons/Beam_Arrays_Light;1;.1;3;5}{upgrades/Weapons/Beam_Arrays_Medium;1;.1;-3;5}{upgrades/Weapons/Mount_Enhancements;1;.1;-50;51}{upgrades/Weapons/Turrets;1;.1;1;1}{upgrades/Miscellaneous/SubUnits;1;.1;1;1}{upgrades/Reactors/Light;1;.1;20;6}{upgrades/Reactors/Medium;1;.1;-2;6}{upgrades/Capacitors/Reactors;1;.1;-5;6}{upgrades/Capacitors/Shields;1;.1;-5;7}{upgrades/Shield_Systems/Light;1;.1;20;6}{upgrades/Shield_Systems/Medium;1;.1;-2;6}{upgrades/Armor;1;.1;20;5}{upgrades/Hull;1;.1;20;2}{upgrades/ECM_Systems;1;.1;6;3}{upgrades/Repair_Systems;1;.1;-2;4}{upgrades/Radar/Basic;1;.1;20;6}{upgrades/Radar/Intermediate;1;.1;9;8}{upgrades/Radar/Advanced;1;.1;-12;17}{upgrades/Miscellaneous;1;.1;2;1}{upgrades/Ammunition;1;.1;52;28}{upgrades/Engines/Engine_Enhancements_Light;1;.1;3;2}{upgrades/Engines/Engine_Enhancements_Medium;1;.1;0;2}
Lets look at the grain entry.
Grain;1.0;0.54;61.75;43 The first number (1) is a multiplier. The base price times this multiplier gives the average price for that base. In this case, the average price of grain on an agricultural world is grain's base price (ag worlds are the cheapest place to buy grain). This number would be higher on a mining base, reflecting its higher price there. If this number is 0, it means that item cannot be sold here (uranium can't be sold on an ag world, so its first number is 0)
The second number (.54) is the percentage by which the price can fluctuate from the average. .54*13 = 7. The price can therefore be anywhere from 7 below 13 to 7 above 13, which matches the values you see in cargo.csv for grain on an ag world: from 6 to 20 credits.
The third number (61.75) has to do with the average amount of the item you can expect to be for sale. If this number is 0, the item is not sold at that base. The fourth number (43) is similar to the second, only it's for quantity. The amount of grain available will vary between 61.75-43 to 61.75+43 (19 - 105) though this is also affected by other merchants buying and selling.
You need those four numbers for each cargo type for each base to get the whole thing to work. It isn't particularly fun to mess with
Maybe someday we'll get it to read something more like cargo.csv.